1. Field of the Invention
The present invention relates generally to a timepiece movement, and more particularly to a timepiece movement having a second stop device wherein a stepping motor serves as a drive source to continuously rotate a second hand wheel substantially in a smooth manner.
2. Background Information
The mechanism of a quartz timepiece movement is such that the rotation of a driving motor is transmitted to a second hand wheel through a gear train to rotate the second hand wheel, and the rotation of the second hand wheel is usually reduced for rotating a minute hand wheel whose rotation is in turn reduced for rotating an hour hand wheel. A stepping motor having a very low battery power consumption is employed as the driving motor. The stepping motor undergoes an intermittent rotary movement, and the second hand wheel is, therefore, designed to undergo intermittent motion at intervals of a second. However, consumers have long desired a timepiece movement in which the second hand wheel undergoes a sweeping motion to enable an indicating hand connected to the second hand wheel to indicate the progress of time continuously.
A sweeping motor has been employed in timepiece movements for realizing a sweeping motion of the second hand wheel. However, sweeping motors have the drawbacks of being expensive to manufacture and having a high battery power consumption.
Proposals have been made for use of motion converting means for imparting a continuous rotary movement to the second hand wheel in the mechanism of a timepiece movement employing a stepping motor as a source of driving force (see, e.g., Japanese Utility Model Application KOKAI No. Hei 2-128994). The proposals include transmitting the rotation of a stepping motor through a gear fixed to a bush fitted loosely about a second hand shaft, and imparting continuous rotation to the second hand shaft by a motion converting means provided in a rotation transmission path and located downstream of the bush.
The proposed motion converting means includes, as a first means, a disk positioned above the bush fitted loosely about the second hand shaft and surrounding the bush rotatably with the second hand shaft, while a spirally wound spring is connected between the bush and the disk to absorb a variation in velocity due to intermittent rotary movement.
As a second means, the motion converting means includes a plate-like member fitted loosely about the leading end of the second hand shaft and connected to the disk by a spring so as to be driven by the disk and thereby absorb a difference in velocity due to intermittent rotary movement.
As a third means, the motion converting means includes a viscous fluid, such as a lubricant, filling a tightly sealed container in which the plate-like member is held, so that the viscosity resistance which results from the rotation of the plate-like member in the viscous fluid imparts a smooth continuous rotary movement to the second hand shaft.
The following two stop means are generally known as second stop devices for use in a quartz timepiece movement.
A first stop means mechanically stops rotation of a wheel in a timepiece wheel train. As shown, for example, in FIG. 5, rotation of a rotor 41 is stopped by mechanical means. A stator 43 is mounted to a lower plate 42. The rotor 41 is rotatably mounted to the magnet section of the stator 43. A drive wheel 44 is meshed with the rotor 41. A rotor stop member 45 is reciprocally moved on an upper plate (not shown). The rotor stop member 45 includes a locking arm 45a for engagement with and disengagement from a rotor pinion 41a which is mounted to the rotor 41, a click arm 45b resiliently in contact with a click post 42a which extends from the lower plate 42, a tongue 45c guided by a guide post 42b which extends from the lower plate 42, and a reset knob 45d extending upwardly through an opening of the upper plate and operable from outside of the upper plate. When the rotor stop member 45 is moved to the right in FIG. 5, the leading end of the locking arm 45a is brought into engagement with the rotor pinion 41a to mechanically stop the rotor pinion 41a.
A second stop means electrically stops rotation of a rotor. As shown, for example, in FIG. 6, a drive circuit of a motor is reset whereby its output is stopped. A lower plate 51 has a pin 51a to which a reset contact element 52 and a printed board 53 are fitted. The printed board 53 has an opening through which the pin 51a extends. An upper plate 54 has a cylindrical post 54a in contact with the printed board 53 adjacent to the opening. A stepping motor includes a drive circuit (not shown). The drive circuit has a reset pattern 53a which is formed on one side of the printed board 53 adjacent to the upper plate 54. The reset contact element 52 has a reset contact portion 52a at its front end.
One end of the reset contact element 52 extends upwardly through an opening 53b of the printed board 53. When a rotor (not shown) is rotated, the reset contact portion 52a is spaced from and faces the reset pattern 53a. A reset member 55 is reciprocally moved on the upper plate 54. The reset member 55 has a press projection 55a which extends inwardly through an opening 54b of the upper plate 54. The other end of the reset contact element 52 is connected to a button battery 56 which serves as a power source for the drive circuit of the stepping motor. A battery lid 57 is attached to the upper plate 54 to prevent the button battery 56 from being moved out of the device.
When the reset member 55 is moved to the right in FIG. 6, the leading end of the press projection 55a urges the reset contact element 52 in a downward direction. This causes the reset contact portion 52a to come into contact with the reset pattern 53a, whereby the drive circuit is reset to stop its output and, therefore, stop the rotation of the rotor.
However, if the foregoing stop means is simply incorporated into a conventional timepiece movement which includes a coil spring and a stepping motor serving as a drive source to continuously rotate a second hand wheel substantially in a smooth manner, the second hand is subjected to extraordinary movement before or after it is stopped.
More specifically, with the prior art stop means for mechanically stopping rotation of the rotor or other wheel in the timepiece wheel train, if, for example, a wheel train upstream of the coil spring is stopped, the second hand cannot immediately be stopped until the coil spring is completely unwound after the wheel train is stopped. Conversely, if the stopping operation of the rotor or other wheel in the wheel train is terminated, the second hand cannot be moved until the coil spring is rewound to a predetermined amount, or the second hand may be moved, but at a speed less than the normal speed. Also, when a wheel train mounted downstream of the coil spring is stopped, the rotor is moved to the extent that the coil spring is wound. Accordingly, the second hand is rotated at a speed greater than the normal speed due to undue winding immediately after mechanical stoppage is terminated.
With the stop means for stopping the output pulse of the stepping motor to electrically stop the rotor, the second hand cannot be stopped immediately since the coil spring is only gradually unwound.